Electrically resistive crucible

ABSTRACT

An electrically resistive carbon crucible for an impulse or resistance furnace designed to have improved heating characteristics.

0 United States Patent [151 3,636,229 Sitek et al. [4 1 Jan. 18, 1972[54] ELECTRICALLY RESISTIVE CRUCIBLE [56] References Cited [72]Inventors: George J. Sltek, Stevensville; Robert N. UNITED STATESPATENTS Reveal, Berrien, both of Mich. v

, 1,015,091 1/1912 Simpson ..13/22X 1 Assignoe: Laboratory EquipmentCorporation, 1,430,858 10/1922 Speirs ..13/22 Joseph, Mich. 1,551,8399/1925 Nash. 13/22 X [22] Filed: Oct 26 1970 2,930,602 3/1960 Rohn...263/48 [21] Appl. No.: 83,786 Primary ExaminerBemard A. GilheanyAssistant Examiner-R. N. Envall, Jr. 521 US. Cl ..13/25, 263/47 Save[51] Int. Cl. .F27d 11/02, 1105b 3/10 [58] Field ofSearch 3/20,22,25,3l;263/47, [57] ABSTRACT An electrically resitive carbon cruciblefor an impulse or resistance furnace designed to have improved heatingcharacteristics.

2 Claims, 3 Drawing Figures ELECTRICALLY RESIS'I'IVE CRUCIBLE BACKGROUNDOF THE INVENTION In the analysis of metals for such components asoxygen, hydrogen, nitrogen, etc., it is common to raise the metal toextreme elevated temperatures to fuse it and release the gaseous orvolatile matter for subsequent analysis. Such technique is applicablegenerally to any composition. Induction heating has frequently beenemployed in the past to obtain this elevation of temperature. Now,however, resistance heating (DC) or impulse heating (AC) is preferred.Either of these consists of passing an extremely high current through acarbon or graphite crucible contained between water cooled electrodes.The crucible constitutes a resistance element which achieves the desiredtemperature of 2,700 C. or higher. The crucible not only constitutes theresistance element in such a usage; it also provides the reactivematerial in the event that oxygen is the subject of the analysis. Uponheating of the sample, the oxygen will react with the carbon of thecrucible to fonn carbon monoxide which may be swept from the furnace bya nitrogen stream for subsequent analysis.

The crucible hitherto employed for this purpose has been a simplecylinder having an axial bore extending partially therethrough from oneend so as to leave a relatively heavy base. The heating in a resistancedevice will, of course, be greatest where the section is smallest.Therefore, with the conventional crucible, the maximum heating willoccur in the wall portion-of the crucible above the base. Since,however, there is heat loss to the cooled electrodes, the areas ofhighest temperature will be about midway along the length of thecrucible.

To withstand the clamping of the crucible between the electrodes forsatisfactory conductivity and to withstand handling generally, such acrucible must obviously have a certain minimum wall thickness. With anincrease in the diameter of the crucible, therefore, such as might bedesired for the analysis of larger samples or solid samples ofappreciable length, the area of the minimum section will increase andthe heating effect be diminished for any given current. Thus, with agiven power supply, there is a highly restrictive limit to the diameterof the crucible employed. The time for heating could, of course, beextended, but this is undesirable in such analytical apparatus, and byvirtue of the heat-sinking effect of the electrodes, even the extensionof heating time will run into a limit.

Also, since the heat is generated primarily in the walls, the floor ofthe crucible must receive its heat by thermal conduction from the walls.At the same time there is the heat loss to the electrodes. The lattereffect can be minimized by making the crucible bottom thick, but as thediameter of the crucible increases, the loss to the electrode becomesproportionately greater to a point where, again, the temperatureachieved at the floor is inadequate.

SUMMARY OF THE INVENTION This invention teaches a carbon crucible havingan integral stud on the exterior of the bottom thereof through which thecrucible is connected into the electrode circuit whereby the sizelimitations of the conventional crucible discussed above are avoided andwhereby the point of maximum heating occurs on the bottom of thecrucible rather than a distance up the sides thereof.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical, somewhat diagrammatic,section through an impulse furnace adapted to use the graphite crucibleof the invention, shown with a crucible mounted therein;

FIG. 2 is a vertical section through the crucible of FIG. 1 taken alongthe line 2-2 of FIG. 1; and

FIG. 3 is a somewhat diagrammatic representation of the section of FIG.2 illustrating the zone of maximum heat.

2 DESCRIPTION OF A PREFERRED EMBODIMENT In FIG. 1 there is shown aportion of combustion apparatus to be used in conjunction withanalytical equipment, the portion illustrating an impulse furnaceparticularly. The furnace includes a stationary top electrode 12 and alower support electrode 14 movable toward and away from the top. Thesupport electrode is cup-shaped with an annular rim 16 and a post 18extending upward from the center of the bottom of the cup, defining anannular groove 20 between the rim and the post. The post is hollow as at22 to provide for a flow of liquid coolant therethrough. The upper endof the post has a cuplike socket 24 in the center thereof, and from thesocket, the upper end slopes broadly conically downward as at 26. Acrucible 28, the subject of this invention, will be mounted to the lowerelectrode post 18 with the stud 30 thereof received in the socket 24.

The top stationary electrode 12 is likewise cup-shaped, but downwardlyfacing. It includes a rim or skirt 32 fitting closely telescopicallywithin the rim 16 of the lower electrode, and 0- rings 34 embedded inthe outer periphery of the skirt make a gastight seal with the rim 16.The top electrode has a large diameter passage 36 extending upward fromthe cup floor 38 for sample introduction and carrier gas admission. Thepassage 36, while large, is smaller than the mouth of the crucible 28.Radial grooves 40 are formed in the floor of the electrode extendingfrom the passage 36 to the upper end of the skirt 32. Liquid passages 42are provided within the body of the electrode for the flow of coolant. Agas exhaust duct 44 extends from the bottom face of the skirt 32upwardly through the electrode to be connected to analysis apparatus.

The crucible 28 is formed of a short length of cylindrical graphite rodwhich is machined to provide an interior cavity 46 defined by upstandingwalls 48, a base 50 somewhat heavier than the walls, and the cylindricalstud 30 projecting centrally from the bottom 50 of the crucible. Thestud will be received in the socket 24 of the support electrode asstated above.

The electrodes are connected across a source of low voltage high currentpower 52 and desirably are made of copper.

The relation of parts in the furnace is such that, when the supportelectrode 14 is raised to its uppermost position with the crucibleattached for sample heating, the top edge of the crucible walls 48engages the floor 38 of the top electrode 12 concentrically with the gaspassage 36. The crucible is thus clamped under pressure between thesupport electrode 14 and the top electrode 12. In such position, thesleeve 32 slides telescopically into the groove 20 but stops short ofthe bottom of the groove 16. The O-rings 34, as stated above, effect agas seal between these telescoping surfaces and likewise serve to spacethe sleeve from the rim 16 in electrically insulating relationship underthe low voltage of heating. The sleeve 32 is also thin enough such thatit stands well clear of the center post 18 of the support electrode.Since, as illustrated, the crucible has a smaller diameter than thecenter post, it will be appreciated that raising the crucible intoheating position will define an annular chamber about the outside of thecrucible.

When the support electrode is in raised position, the bottom edge of theskirt 32 stands well above the bottom of the groove 20, and the outletpassage 44 is thus in communication with the above-defined annularchamber. The radial grooves 40 in the top electrode open the interior ofthe crucible to the outside, annular chamber.

In operation, a sample will be placed in a conventional loading deviceabove the passage 36, not shown, and there purged by a flow of carriergas. At the same time, the crucible will be heated in the same gasstream to eliminate occluded and absorbed gases. Thereafter, the samplewill be dropped by the loading device into the crucible and the crucibleheated to fuse the sample. Evolved gases from the sample will be sweptby the carrier gas stream, entering through passage 36, through radialgrooves 40 and out the gas outlet 44 to analyti' cal apparatus.

A representative crucible made in accordance with the teachings of thisinvention will have an outside diameter of 0.56 inch and a wallthickness of 0.04 inch. The floor of the crucible has a thickness of0.12 inch, and the stud has a diameter of 0. l 87 inch and a projectionof about 0.13 inch. The stud is filleted at its juncture of the bottomof the crucible 50 for reasons of strength. The socket 24 is 0.08 inchdeep, thus leaving a separation of 0.05 inch between the bottom of thecrucible and the upper end 26 of the post 18.

As is common with carbon conductors, some pressure is necessary to makea good conductive contact with an electrode. Since the crucible is heldbetween the top edges of the walls and the base of the stud 48, primaryconduction will be through the bottom surface 54 of the stud. The studof the crucible detailed above has a cross section of about 0.025 squareinch. The walls have a section equal to about 0.06 square inch.Therefore, with the passage of a high amperage current through thecrucible, the stud will ofi'er the greater resistance and will heat to acorrespondingly higher degree. The heating, of course, will be sinkedaway to a degree by the water cooled electrodes, but by thermalconduction, the floor 46 of the crucible will heat to the greatestdegree as particularly illustrated in FIG. 3. The areas of pressurecontact, as well as providing conductive contact, are also the principalareas of heat loss to the electrodes. The increment of stud nearest thebottom of the crucible will therefore be the hottest portion of thesystem. This is in notable contrast to the previously employed studlesscrucible, where the outer periphery of the base of the crucible is incontact with the lower electrode (through a tripodal electrodeconfiguration), current flow is primarily through the annular portion ofthe base aligned with the walls, and wherein the point of maximumheating occurs about halfway up the walls of the crucible.

By virtue of the heating of the floor of the crucible, the diameter ofthe crucible can be made substantially larger and thus be receptive tolarger samples including pin samples which can lie flat on the floor ofthe crucible rather than be held in an inclined position therewithin aswould be the case with the smaller diameter crucible.

The stud not only provides a small section conductor whereby the heatingeffect is augmented; it also serves as a stand-off insulator wherebycontact of the base of the crucible with the water cooled electrode isavoided thus preventing loss of heat from the base to the electrode.Also, since the floor alone is the matter of primary interest in thecrucible heating, there is less bulk of crucible to be outgassed, andaccordingly, the outgassing time may be substantially reduced.

We claim:

1. An electrically resistive graphite crucible for fusion apparatushaving cylindrical sidewalls, a bottom, and a stud projecting downwardlyfrom said bottom, said stud having a cross-sectional area less than thecross-sectional area of said wall.

2. The crucible as defined in claim 1 wherein said stud has across-sectional area less than half the cross-sectional area of thewalls of said crucible.

Disclaimer 3,636,229.-Ge01-ge J. Sitelc, Stevensville, and Robert N.Revesz, Berrien, Mich. ELECTRICALLY RESISTIVE CRUCIBLE. Patent datedJan. 18, 1972. Disclaimer filed J an. 12, 1978, by the assignee, LecoOorporatz'on.

Hereby enters this disclaimer to claims 12 of said patent.

[Ofiicial Gazette Mamh 21, 1978.]

Disclaimer 3,636,229.Ge0rge J. Sitelc, Stevensville, and Robert N.Rewesz, Berrien, Mich. ELECTRICALLY RESISTIVE CRUCIBLE. Patent datedJan. 18, 197 2. Disclaimer filed Jan. 12, 1978, by the assignee, L600Corporation.

Hereby enters this disclaimer to claims 12 of said patent.

[Oficz'al Gazette Mamh 21, 1978.]

2. The crucible as defined in claim 1 wherein said stud has across-sEctional area less than half the cross-sectional area of thewalls of said crucible.